Field of Invention
The invention relates to systems and methods for cultivating photoautotrophic microorganisms.
Description of Related Art
A photoautotrophic microorganism is an organism that is capable of generating its own sustenance from inorganic substances using light as an energy source. As an example, photosynthetic microscopic algae, hereinafter referred to as algae, is a photoautotroph. Algae are unicellular organisms which produce oxygen by photosynthesis, and may include flagellates, diatoms, and blue-green algae. More than 100,000 species of algae are known.
As mentioned above, algae use a photosynthetic process similar to that of higher-developed plants, with certain advantages not found in traditional crops, such as rapeseed, wheat, or corn. Algae have a high growth rate, and it is possible to complete an entire harvest in hours. Further, algae are tolerant to varying environmental conditions, for example, algae may be grown in saline waters that are unsuitable for agriculture. Due to this tolerance to environment and climate, algae are responsible for about one-third of the net photosynthetic activity worldwide.
As a result, during the past decade, much focus has been aimed at the production of algae for commercial purposes. This focus is evidenced by the manifestation of many new industries and uses of algal production, including but not limited to the following: (i) use of algae as a source of fatty acids, proteins and other bio-chemicals in the production of nutraceuticals, health food, food additives, vitamins, pharmaceuticals, and natural dyes; (ii) use of algae as an animal feed supplement with nutritional value equivalent to that of soybean meal; (iii) use of algae as a biological control of agricultural pests; (iv, use of algae as soil conditioners and bio-fertilizers in agriculture; (v) use of algae for the production of oxygen and removal of nitrogen, phosphorus, and toxic substances in sewage treatment; (vi) use of algae in the bio-degradation of plastics; (vii) use of algae as a renewable biomass source for the production of a diesel fuel substitute (biodiesel) and other biofuels such as ethanol, methane gas, and hydrogen; and (viii) use of algae to scrub CO2, NOx, VOx from effluent released during the production of fossil fuel. With so many uses, it would be desirable to mass produce algae in a low-cost, high-yield manner.
One commercial purpose of significant import includes algae cultivation as a renewable biomass source for sustainable biodiesel production. Presently, the renewable biomass source is provided by edible oils, such as soybean oil, palm oil, and rapeseed oil. It is of interest to develop additional types of renewable biomass sources, such as lipids from algae.
As briefly noted above, one factor making algae interesting as a renewable biomass source is that algae may be grown under conditions or in places not suitable to other sources. Accordingly, algae may be grown and used in ways that do not significantly compete with food sources or agriculturally productive land.
An additional factor making algae of interest is the availability of material to convert to biodiesel or other fuel. Some algae have a lipid content as much as 50% to 70% of their dry weight. By way of comparison, the lipid content in dry soybeans is approximately 20%. Algal lipids have a similar composition to vegetable oil and are readily adaptable as a renewable biomass source to existing biodiesel manufacturing processes. Further, the remaining algal biomass may be converted to bio-ethanol, converted to biodiesel, converted to methane, burned, or used as food for other organisms.
Another commercial purpose of significant import includes algae cultivation for the production of omega-3 oil, omega-6 oil, and axstasanthin (3,3′-dihydroxy-β-carotene-4,4′-dione) in pharmaceuticals, nutraceuticals, and food supplements.
Commercial acceptance of biomass products is dependent on a variety of factors such as, for example, cost to manufacture, cost to operate, reliability, durability, and scalability. Commercial acceptance of biomass products is also dependent on the ability to increase biomass product growth and recovery, while decreasing biomass production cost. Therefore, it may be desirable to have novel approaches for growing and harvesting biomass products including, for example, cell components such as lipids, proteins, vitamins, fatty acids, minerals, carotenoids, pigments, and the like.
Providing faster growth and producing high density cultures is critical to achieving the operational scale necessary for current environmental and industrial needs. Ideally, improving the speed of growth and increasing the density of a culture will require less production space and consequently will lower the cost of associated facilities.
In line with these reasons and others, the cultivation of algae in liquid suspension, rather than within the ground, allows greater access to the nutrient sources necessary for growth, i.e., water, CO2, and minerals, and permits reducing the production space to a cost-effective footprint. Accordingly, considerable activity has been focused on efficiently growing photoautotrophic microorganisms in liquid suspension, and specifically to mass culture unicellular algae.
Algal yield can be restricted by the limited wavelength range of light energy capable of driving photosynthesis, between about 400-700 nm (nanometers), which is only about half of the total solar energy. Other factors, such as respiration requirements during dark periods, efficiency of absorbing sunlight, and other growth conditions can affect photosynthetic efficiencies in algal bioreactors. The net result is an overall photosynthetic efficiency that has been too low for economical large scale production. Thus, the need exists for a large scale production system that provides the user a cost-effective means of installation, operation and maintenance relative to production yields. It is desirable that such a system also increase photosynthesis to maximize production yield.
In order to produce optimal yields, algae need to have CO2 in large quantities in the basins or bioreactors where they grow. In addition to CO2, the growth rate of algae may benefit from exposure to other nutrients that are common in known plant fertilizers.
Furthermore, algae need effective control of light. To maximize the growth of photosynthetic organisms, light must be available at the right intensity, the right frequency, and without excessive heat. Excessive light intensity can limit growth by inducing photo-respiration or bleaching the pigments needed for efficient cell growth. In addition, light intensity or light frequency in excess of the culture requirements may result in heat build-up that can limit culture growth. These problems are readily apparent in production systems that rely solely on direct solar light as a driver of photosynthesis, such as in ponds and raceways. Solar light is subject to extreme diurnal and seasonal variability. In addition, cultures relying on direct solar light are subject to periodic heating from light intensities and spectra not immediately useable by the culture.
The ability to effectively control light for 24 hours per day encourages faster growth of biomass and secondary metabolites as well as continuous consumption of CO2. This results in a more efficient use of facilities, enabling a smaller footprint for a given level of production. However, the use of artificial light has a cost that must be minimized for successful industrial application.
The two major obstacles that reduce the uniform delivery of light to a culture are turbidity and bio-filming. Turbidity occurs as a culture approaches a density where some of the organisms shade others from the light. Ensuring delivery of the appropriate amount of light to each organism becomes increasingly difficult as the density of the organisms in a culture increases. Turbidity within a culture results in some organisms receiving less light than they can use and non-productive absorption of light by other organisms. The absorption of excess light wastes energy and contributes to heat build-up.
Bio-filming is an extremely widespread problem that occurs when a microorganism adheres to a surface. Most microorganisms, including species in all three domains, i.e., bacteria, eukaryotes, and archaea, perform processes that result in adhesion to surfaces and to other microorganisms. In industrial applications, biofilms often clog or corrode pipes and surfaces. In photo-bioreactors a biofilm can form over a light-delivery surface, thereby reducing the intensity and changing the spectra of the light transmitted.